The present invention relates to a fastener assembly and, more particularly, to a fastener assembly of the type including a bolt and a washer combination adapted for use in securing one member to another in an environment where the bolt and washer are subjected to vibration and wide-range temperature variations which tend to cause the tightened assembly to loosen. Typically, the fastener assembly may be used in securing the exhaust manifold of a combustion engine to the engine block. Examples of fastener systems designed for use in environments of this nature are disclosed in U.S. Pat. Nos. 3,275,055 and 3,352,344. Another fastener system which might be considered to be relevent to the invention claimed herein is disclosed in U.S. Pat. No. 2,297,957.
In an environment of the foregoing nature it is desirable to achieve a high bolt load to tightening torque ratio so that as much as possible of the torque applied to the bolt in securing it to the engine block is converted into bolt load, that is, the axially directed, internal force of the bolt which holds the manifold on the block. Also, it is desirable to achieve consistency in the bolt load from one bolt to the next when several fastener assemblies are tightened to the same torque in securing the manifold to the engine block. Moreover, it is desirable that the breakaway torque, or torque required to loosen a tightened bolt in the block, be as high as possible in relation to the tightening torque and that the bolt and washer assembly be constructed so that, even with annealing of the bolt and washer due to heat in the manifold, the breakaway torque remains high to keep the assembly from being loosened easily.